PLATE 25. 



Polarised Light. 



A CRYSTALS OF CINCHONIDINE WITH GROSSED NICOLS x 45 

 B SECTION OF QUARTZ DIORITE WITH GROSSED NICOLS x 20 



Objective 



Ocular 



Condenser 



Illuminant 

 Aux. Condenser 



Screen 



Plate 



Exposure 



Developer 



A B 



Leitz No. 3 Achromat. Grouch 2" Achromat. 

 16mm. N. A. 0.3. 



Leitz 0. Leitz 0. 



Back combination of Watson's Universal Achro- 

 matic, placed above the polariser. 

 Inc. gas, inverted. Liliput 4 amp. arc. 



Bull's-eye, covering the lower face of the 

 polariser with a convergent beam. 



Wratten'sK 1 

 Wratten's"M"Pan. 



2min. 



Wellington 



"Anti-Screen." 



5 sec. 



Pyro-soda. 



An endeavour is generally made to reproduce the appearance of 

 polariscope objects as closely as monochrome tones allow. The yellow 

 screens K x and K 3 were used with this purpose in view, they were 

 selected in accordance with the conditions of light and plate in 

 conjunction with the colours of the specimen. When reds and 

 yellows largely predominate a Panchromatic plate must be used, but 

 an Ortho. plate is quite satisfactory where blues and greens are the 

 chief colours. 



If it is desirable in any case to emphasise one colour in contrast to 

 the others suitable screens can easily be selected. 



Photos. W. B. R. 



\To follow page 188. 



